Edinburgh – home of the Scottish Parliament, Military Tattoo, Princes Street and gardens, Scott memorial, Murrayfield, Valvona and Crolla’s food emporium, sundry pubs (!), the fringe, volcanoes … eh, volcanoes?
Surprising as it may be to some people, Edinburgh plays host to a great variety of igneous rocks. The most obvious, and in our case the most interesting, are the volcano remnants of Arthur’s Seat and the Castle Rock. These are long extinct and date from the Lower Carboniferous, over 300 million years ago. Arthur’s Seat, the larger of these, is assumed to be the main structure, with Castle Rock a subsidiary or satellite (or, if you prefer, parasitic) vent. These are central pipe vents from pipe conduits, c.f. linear conduits as associated with fissure eruptions.
The rock of Arthur’s Seat is mainly a vent agglomerate with several crystal phyric microgabbro eruption pipes (Lion”s Head and Lions Haunch vents are so called as at a distance the remnants resemble a lion lying down). These are difficult words – let’s explain what they mean. An agglomerate is a debris mix, part explosive and part vent collapse, of ash and lava. Crystal phyric refers to the presence of some minerals present with larger crystals than the background rock. The presence of gabbro in the conduit pipes indicates these volcanoes erupted a basaltic lava: gabbro has the same mineralogical composition as basalt but is intrusive (did not erupt) and is coarse grained. Micrograbbo, Dolerite, or Diabase to some authors, has the same composition, but has a grain size intermediate between basalt and gabbro, smaller than a grain or rice. The magma that forms gabbro and micrograbbo is rich in iron and magnesium, and poor in silica. The grain size depends on how quickly it cooled. Slower cooling causes larger grains. Gabbro, with the largest grains, cooled very slowly in larger magma chambers. Micrograbbo, with intermediate grain size, cooled in dyke and sill intrusions. The combined vent material of Arthur’s Seat, as mapped, gives an irregular vent of 750-1000 metres across.
The picture of pipe brecciated St Austell granite illustrates the general appearance of vent agglomerate; however, the detail is different since a true vent agglomerate has fragments with a much larger range in both size and composition.
The opening picture shows the main vents of the Lion’s Head (left) and Lion’s Haunch (right) in the background, with Salisbury Crags in front; these are the quarried remains of a post volcanic episode Teschenite: an olivine analcime microgabbro – analcime is a hydrous sodic zeolite mineral comparable to feldspathoids; zeolite minerals are aluminosilicate, and are usually associated with empty bubbles (vesicles) in amygdaloidal basalt flows (amygdaloidal means that the basalt contains holes or bubbles), which are later filled with minerals deposits). Teschenite has a lowish olivine content of 5-15%. It is common in Scotland, and in this case it formed when a sill intruded into sub-volcanic sediments of Lower Carboniferous age.
Salisbury Crags are part of a site dedicated to James Hutton, who has been called the ‘Father of Geology’. It was here that Hutton part formulated his theory of Uniformitarianism, that confounded the Neptunism movement that insisted the Earth dated from the biblical Great Flood, by siting the presence of a rock of obvious molten origin being intruded into sediments/volcanic rock, the sediments being just visible below the sill.
Away from the volcano, mapping and borehole evidence shows that there are up to 250 metres thickness of tuffs and lavas adjacent to the vent and in the Midlothian borehole. Some 10 km to the south east, approximately 70 m of volcanic material was found at the horizon of the Arthurs Seat volcanics.
Castle Rock is the erosional core of a relatively small parasitic volcano off Arthur’s Seat, composed of microgabbro. It is approximately 150 metres in diameter. The vent again cuts through shallow water marine sediments of Lower Carboniferous (Dinantian) age. These sediments comprise predominantly sandstones and minor shale horizons; but it should be noted that in some texts the sediments erroneously are noted as limestones, a confusion arising from the rocks of this age being assigned to the Carboniferous Limestone division.
More recently, during the last Ice Age, ice sheet movement has produced a classic example of a ‘crag-and-tail’ with the Castle Rock – the crag – protecting the Carboniferous sediments of the Royal Mile – the tail – from ice erosion, indicating that the mass of ice came from the west. More recently, the ice-deepened gouge channel on the north side (Princes Street gardens) has been utilised by the railway as an ideal route through the city! It is kind of appropriate – trains in the UK have the reputation of ‘running’ at a glacial speed.
So what happened? For this, we need to go back to the landscape of 350 million years ago. This part of Scotland had a warm, wet climate (half of which hasn’t changed). It was located a little south of the equator. There were mountains to the north, and to the south was sea (England was submerged and even wetter than Scotland). Rivers brought sediment from the mountains and deposited them on the coastal plains. The coast moved back and forth as sea levels changed. This created layers of different sediments. Sometimes there were swamps with dense vegetation, sometimes there was only river sand. The petrified sand was later used for the buildings of Edinburgh, while the swamps became Scotland’s coal fields. And at times the sea rose so much that all of lowland Scotland was under water, and a layer of limestone formed.
There had been another ocean here before, the Iapetus ocean which closed and went out of business about 400 million years ago. That closure had brought Scotland and England together, in a pre-historic Act of Union, and had pushed up the mountains north of Edinburgh. But the ocean that was now to the south was a different one; it separated the newly merged post-Iapetus land (Scandinavia, England (Avalonia) and North America) from Gondwana. This was the Rheic Ocean, and it too was on its last legs. About 320 million years ago the Rheic Ocean went extinct and Pangea was born. England is a graveyard of oceans, with Scotland playing -not entirely successfully- the innocent onlooker.
There was 70 million years between the closing of the two oceans. That is quite a lot of time. The creation of Edinburgh took place in this period of time between the two oceans. The closing of the Iapetus ocean involved an subduction zone to the northwest, and subduction causes volcanoes – often explosive ones. So it was here. South of Edinburgh are the Pentland hills, running from Biggar (a nice little market town with a small museum that is well worth a visit) towards the northeast, to Edinburgh. These hills started out as Iapetus ocean floor, covered with kilometers of sediment and pushed up in the continental collision. Now volcanoes erupted on top of this: they added kilometer-thick lava and ash layers, ranging from basaltic to rhyolitic.
As the Rheic ocean began to close, England found itself in an area of subsidence. Much of England went under water. The Pentland hills were now on the shores of an ocean they had nothing to do with. The closest analogy I can think of is Cape Town which claims to have two oceans, one on either side. Cape Point, at the tip of the peninsula that also contains the Cape of Good Hope, can be an amazing place. If you like plants, you could spend a week here and see a different erica every hour. Most of the visitors ignore them and go straight to the vertiginous look-out point. Out at sea, sometimes you can clearly see the difference between the two currents coming from the two oceans. And sometimes you can’t: the currents don’t always meet at the same place. Do be aware of the tourists, your wallet, the incessant wind, and the baboons. (The latter are the most sexist animals I know, attacking women tourists (and children) but leaving the men alone. Made me think of some humans.) Edinburgh was like this, apart from the pesky detail of 70 million years between the two oceans. The climate was hot, far hotter than Cape Town – some of the limestone deposit in England indicate mean annual temperatures over 30 C. Think Persian Gulf.
The closure of the Rheic ocean caused a new phase of volcanic activity. This was different from before, and it was not directly subduction driven. Perhaps a graben tried to form. It has also been suggested that it was triggered by the subduction of the old spreading center of the Rheic ocean, with its residual heat. Small centers of volcanic activity developed where basalt came up, forming relatively small, short-lived volcanoes. These form a line across the midland valley of Scotland, from the coast near Edinburgh running in the direction of Stirling. The eruptions happened in a shallow sea and they build up small islands. Where lava met sea water, explosions could happen, but otherwise the activity was effusive. The volcanoes themselves have long since eroded away. Only the central plugs have survived: Arthur’s seat, Castle rock and Stirling castle are examples; East Lothian (east of Edinburgh) has more examples. The area around Arthur’s Seat shows evidence for 13 different lava flows, which is not a huge number seeing it came from three separate vents. Of course, other flows may have eroded away.
An area further west, around Glasgow, experienced volcanic activity at about the same time. This was larger in size, and it led to the formation of the Clyde Plateau lava extending northeast from Glasgow towards Stirling. (This poor town had to defend itself against volcanics encroaching from two sides – luckily it is a well-defended citadel.) Originally the Clyde Plateau may have covered 3000 km2. The Renfrewshire Hills are a remnant of this: they form a large lava plateau, probably fed from fissure eruptions, with numerous dikes well below the surface. From Dumbarton onward the flows become vent-driven: Dumbarton rock belongs to this sequence. The hill of Meikle Bin is perhaps the most impressive of these vents, and it may have been part of a caldera. There are other volcanic fields further west but these are younger. If you are in the mood to visit Glasgow ( a badly underrated city): the famous Necropolis (with 50,000 inhabitants, all deceased) is build on a small hill next to the cathedral: it formed from an intrusion in this younger Permian volcanics. Rumour has it that a new Batman film is being filmed on the Necropolis.
As this brief phase of volcanic activity waned around Edinburgh, the magma became too sluggish to reach the surface. Instead it spread out in large underground sills, and over time slowly solidified. The cooling allowed grains to grow, and the end result was that layers of dolerite formed. When you walk on the old streets of Edinburgh (a much recommended activity, trying to select your favourite pub – best done by trial and error) (although I could not find any errors), the stones under your feet come from this dolerite, quarried in various places around the city. Arthur’s seat is dated to 342 million years ago whilst the dolerite is 325 million years old.
And now the Rheic ocean finally closed. This seems to have been a relatively relaxed affair, which did not form high mountains. But the land was pushed up and Scotland became dry, now safely embedded in the heart of Pangea. The extinct volcanoes quietly waited for the arrival of the Scots, and the building of Edinburgh.
Walking around in Edinburgh is a rewarding geological and volcanological activity. It is a beautiful city, and a lively one. It is also small enough that everything is within walking distance. All around you are the memories of its volcanic past, fed by interactions with all three surrounding continents. This land has held its volcanic (red-haired) head high for the past 300 million years. The Scots picked a good place to live.
ALAN C & Albert
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And finally, can you identify the mystery stone?